Thermal action cleaning devices are generally very efficient and require little space. Their main drawback is their high energy consumption, an energy which is necessary for bringing the gases to be processed to the oxidation temperatures (850.degree. C. to 1100.degree. C.), a drawback which is decreased if the cleaning is performed in the presence of catalysts at much lower temperatures (200.degree. C. to 450.degree. C.).
For evident economic reasons, it is necessary, in all cases, to recover the greatest part possible of the heat accumulated by the effluents while passing through the thermal cleaner by means of thermal exchangers located downstream therefrom. In the case of an incineration in the presence of a catalytic bed, the effluents are heated prior to their incineration by passing into another thermal exchanger located upstream. The overall thermal efficiency depends on the effectiveness of the exchangers. In practice, autothermal incinerators are produced for cleaning gases laden with at least 0.7 g/m.sup.3 of air.
A well-known heat exchange process consists in circulating the gases to be cleaned between two masses capable of taking up, of storing and of releasing the heat. By crossing the first mass, the effluents heat up until they reach a temperature close to that necessary to the oxidation of the polluting matters. They are then fed into a combustion furnace (with flame or flameless) or in a catalytic bed where they oxidize according to an exothermic reaction. The gases then cross the other mass to which they give up their calories prior to being discharged outside. The direction of flow is periodically inverted.
The main drawback of this periodic inversion is to disturb the processing regularity or its efficiency. Furthermore, it requires the intercalation of valves suited to the effluents pipes of often great section. If one chooses in fact to favour the cleaning quality, any mixing between the polluted and the cleaned gases must be prevented during the cycle inversion periods and the processing must therefore be stopped for a short time interval (some seconds in practice when each cycle lasts for several minutes). If the processing continuity is imposed, the mixing of the flows at the time of the inversions of direction during the intercycles, and therefore a momentary efficiency loss, must be accepted.
Another notable drawback of the heat exchange devices with periodic inversion is due to the fact that the preheating chamber, which is upstream from the furnace during a cycle, is thereafter downstream therefrom during the next cycle. The result of this is, on the one hand, a mixing of polluted and of cleaned effluents in this chamber during the intercycle and, on the other hand, a variation of the chamber temperature during the next cycle.
A well-known technique, notably used in thermal power plants, comprises using a rotary drum of vertical or horizontal axis. The efficiency obtained is relatively low (of the order of 60 to 75%) because the flows of unequal temperatures which exchange heat pass through the drum parallel to the axis thereof and are thus not properly separated from one another in the adjoining zones of circulation.
Another well-known heat exchange technique comprises using a crossed-flow thermal exchanger made with plates or tubes, in which the heated effluents give up their calories continuously to the gases to be cleaned. This technique is costly for average or high flow rates, because of the large heat exchange surfaces it implies and of the care to be exercised in order to obtain a perfect separation of the two flows.